GVR multiview support

[mkeblx]

I saw that latest GVR (1.40.0) added its support for multiview rendering, via OVR_multiview2 extension like Oculus backend. https://developers.google.com/vr/android/ndk/gvr-ndk-rendering#optimize_performance_with_multiview

[thomasflynn]

fwiw, you can enable multiview rendering with GVRf. but we currently only support our 'Texture Shader' with it. Rework to support other kinds of shaders, including custom shaders, is part of the rework that is being done for Vulkan support.

[roshanch]

@thomasflynn multiview support is not limited to texture_shader, custom shaders also have that support.

[thomasflynn]

well then, i stand pleasantly corrected :). So, then you just need to enable it in the gvr.xml file and you should be good to go.

[liaxim]

While multiview support is in the framework and in the Oculus backend (though infrequently tested but that is a different problem) there is some work to be done in the Daydream backend. Plus the daydream backend needs to check the flag from gvr.xml. The parsing of gvr.xml is not centralized but that is yet another different problem.